A retractor has a cylindrical spool. Seat belt webbing is attached to and wound around the spool and the spool is mounted on a spool shaft in the retractor to be rotatable. Webbing is wound onto the spool under action of a retractor spring and is paid out under the influence of relatively gentle forwardly directed movement of a vehicle occupant, for example to allow for normal movement associated with vehicle occupancy such as reaching forwards to activate in-car controls (for a radio or a window) or to reach a glove compartment or door pocket. In the event of a crash, the more extreme momentum of the vehicle occupant activates a crash sensor which locks the spool against rotation and thus prevents forward motion of the vehicle occupant and injury due to the vehicle occupant colliding with the interior fixtures of the vehicle such as the steering wheel, dashboard or windscreen.
One known approach is to interpose a deformable member such as steel torsion bar in the force path between the locking mechanism and the spool. Under application of a high torque a torsion bar can rotate up to 7 or 8 times whilst still remaining intact and thereby allow pay-out of webbing generally in proportion to the momentum of the vehicle occupant at the moment of a crash condition being sensed. It thus reduces the injurious effects of the seat belt in a crash. The load limiting effect provided by such retractors is reliant upon the material properties of the deformable member and so only one predefined level of load limiting can be achieved for a given crash.
The forces exerted on a vehicle occupant during a crash vary. There are higher forces during the initial moments of the crash and the force decreases with time. Thus two-stage load limiting retractors have been suggested. These typically include a combination of two deformable elements arranged such that one deformable element provides continuous load limiting and the second element is engaged at a predetermined point in time to temporarily provide a higher level of load limiting.
Known retractors provide load limiting but do not allow for adjustment to suit different vehicle occupants and crash criteria in situ. The weight and size of a vehicle occupant and the crash severity affects the performance of such load limiters. It would be an advantage to have a retractor that can offer load limiting to suit such variables more closely.